1. Field of the Invention
This invention relates to an optical scanner, and particularly to an optical scanner which causes an optical signal to scan by means of a moving optical reflecting means.
2. Description of the Prior Art
In the conventional optometric measuring instruments known as laser flare meters and the like, a technique of deflecting a laser beam using an oscillating mirror is known. FIG. 1 shows the structure of such a laser scanning system. Here, only the structure used for one-dimensional scanning is illustrated, but by a combination of these structures, two-dimensional scanning is possible.
In FIG. 1, 13 is a semiconductor laser element or other source of laser light; this laser light impinges on a mirror 11. The mirror 11 is driven by a galvanometer 12 or other driving means to oscillate in the direction of arrow 15. Thereby, the laser light reflected by mirror 11 scans as indicated by arrow 16.
In such a structure, in order for the spot illuminated by the laser beam reflected from the mirror 11 to scan repeatedly over a target 14 at constant velocity, ideally the displacement of the angle of oscillation of the mirror 11 must vary as indicated on FIG. 6. If the inertia of mirror 11 and other conditions are ignored, the waveform of FIG. 6 can be considered to be the ideal driving waveform of the galvanometer 12.
The driving waveform of the galvanometer 12 is conventionally generated by an analog circuit as shown in FIG. 7. The circuit of FIG. 7 is a sawtooth-wave generator circuit which uses an integrator based on an operational amplifier. In FIG. 7, the plus input of operational amplifier 71 is grounded while its minus input receives through a resistor R.sub.s a voltage which is obtained by a collector supply voltage V.sub.CC divided by a variable resistor VR.
A positive feedback circuit comprised of a capacitor C and a switch SW (comprised of a transistor or the like) is provided between the output terminal of the operational amplifier 71 and its minus input. When switch SW is turned on and off at a fixed interval t.sub.w by the output of another oscillator or the like, the operational amplifier 71 produces an output voltage V.sub.O having a waveform as shown in FIG. 8. Note that what is actually shown here is a voltage -V.sub.O which was inverted by an inverting amplifier (not shown) or the like.
In FIG. 8, the `on` timing of switch SW is indicated by the symbol c and its `off` timing by the symbol o. During the period in which switch SW is off, the operational amplifier 71 acts to integrate the output voltage across variable resistor VR in accordance with time constants R and C, so that the output voltage V.sub.O increases linearly. On the other hand, when switch SW is turned on, capacitor C is discharged and the output voltage returns linearly to 0 V.
By repeating this integrating action, the driving signal for galvanometer 12 is generated.
However, even if the driving waveform of galvanometer 12 is a sawtooth wave as shown in FIG. 8, the inertia of the galvanometer 12 and mirror 11 will prevent the ideal scanning waveform of mirror 11 as shown in FIG. 6 from being attained, but rather the displacement of mirror 11 will exhibit deviations as shown in FIG. 9. In particular, overshooting appears where the rate of oscillation of the mirror 11 is high, namely in region C of FIG. 9 at the transition from region B (the returning area) to region A. This overshooting becomes more pronounced the greater the mass of the rotating parts of FIG. 1, the higher the velocity of scanning and the greater the rate of change of velocity between regions B and A on FIG. 9.
With such mirror displacement, not only is accurate laser scanning prevented, but there is also a problem of adversely affecting the performance of springs and other parts within the galvanometer 12.
Expensive galvanometers have built-in braking coils which are effective against such overshooting, but inexpensive galvanometers cannot be made to scan accurately without making modifications to the driving waveforms or taking other measures.
For example, by rounding the sharp portions of the driving waveform provided as input to the galvanometer 12 to give a waveform as shown in FIG. 3 and thus lowering the velocity of the mirror 11 when it reverses its motion, overshooting may be reduced. However, with conventional analog circuits, complicated and expensive waveform-shaping circuits are required to generate the signal shown in FIG. 3.
Furthermore, in order to adjust the driving frequency of mirror 11 and the velocity of the mirror 11 when it reverses its motion, the time constants in analog circuits fundamentally must be modified. This makes it impossible to alter only one part of the waveform or to set other extremely exact conditions, thus creating another problem.
The above problems are common to various types of scanning devices which involve optical scanning using mechanical driving means.